1. Field of the Invention
The present invention relates to an optical communication system, and in particular to a wideband amplifier with at least one erbium-doped fiber.
2. Description of the Related Art
Recent exponential growth in the volume of data transmission has necessitated increasing transmission capacity of wavelength division multiplexing (WDM) optical communication systems. There are two possible approaches for increasing transmission capacity: one is to increase the number of transmission channels, and the other is to increase the speed of transmission. Many studies have addressed increasing the transmission speed. There are two possible approaches to increasing transmission capacity: one is to use a parallel connection of a C-band erbium-doped fiber amplifier (C-band EDFA) with an L-band erbium-doped fiber amplifier (L-band EDFA), both of which are already in use, and the other is to use a new amplification medium such as a thulium-doped fiber. However, there are drawbacks to the latter approach in that optical fiber amplifiers doped with rare-earth elements are not only restricted as to available amplification band, but also have a high noise figure. As an alternative, current research is actively being directed to a Raman optical fiber amplifier.
Wavelength ranges in optical communications are generally classified as C-band, L-band and S-band. C-band has a wavelength range between 1530 nm and 1565 nm, L-band between 1565 nm and 1610 nm, and S-band between 1450 nm and 1510 nm.
FIG. 1 illustrates a configuration of a conventional optical fiber amplifier. The optical fiber amplifier includes first to fourth isolators 110, 150, 170 and 210, first and second pumping light sources 130 and 190, first and second wavelength selective couplers 120 and 180, an erbium-doped fiber 140, a connector 160 and a dispersion compensating optical fiber 200.
The first isolator 110 allows optical signals inputted in one direction to pass through but it does not allow optical signals inputted in the other direction, i.e., through the first wavelength selective coupler 120, to pass through.
The first wavelength selective coupler 120 combines optical signals inputted from the first isolator 110 with first pump light inputted from the first pumping light source 130, and then outputs the combined results to the erbium-doped fiber 140. The first pumping light source 130 pumps the erbium-doped fiber 140 in a forward direction, i.e., pumps erbium ions. In a preferred embodiment, the first pumping light source 130 comprises a laser diode for outputting the first pump light at a wavelength of 980 nm. The erbium-doped fiber 140 is pumped in a forward direction by pump light inputted through the first wavelength selective coupler 120, thereby amplifying and outputting optical signals inputted through the first wavelength selective coupler 120. The second isolator 150 allows optical signals inputted in one direction, i.e., through the erbium-doped fiber 140, to pass through but it does not allow optical signals inputted in the other direction to pass through.
The connector 160 connects an erbium-doped fiber amplifying section 220 on the leading side with a Raman optical fiber amplifying section 230 on the following side. One example of the connector 160 is a ferrule with a circular hole in its inner side.
The third isolator 170 allows optical signals inputted in one direction, i.e., through the connector 160, to pass through but it does not allow optical signals inputted in the other direction to pass through.
The second wavelength selective coupler 120 combines optical signals inputted from the third isolator 170 with a Raman pump light inputted from the second pumping light source 190, and then outputs the combined results to the dispersion compensating optical fiber 200.
The second pumping light source 190 performs a Raman-pumping of the dispersion compensating optical fiber 200 in a forward direction. In a preferred embodiment, the second pumping light source 190 comprises a laser diode for outputting the second pump light at a wavelength of 1450 nm.
The fourth isolator 210 allows optical signals inputted in one direction, i.e., through the dispersion compensating optical fiber 200, to pass through but it does not allow optical signals inputted in the other direction to pass through.
The conventional optical fiber amplifier as describe above comprises two amplifying sections 220 and 230. A first amplifying section comprises the erbium-doped fiber amplifying section 220 on the leading side of the connector 160 and the second amplifying section comprises the Raman optical fiber amplifying section 230 on the following side of the connector 160. This double arrangement diminishes the device's price competitiveness as well as increasing its total required volume, thus also diminishing its capacity for integration.
Thus, there is a need for a cost effective wideband amplifier having improved integration capability.